The disclosed subject matter relates generally to voltage regulation and, more particularly, to feed-forward compensation for a low-dropout voltage regulator.
Voltage regulators are used to provide low noise power supplies for sensitive analog devices in an integrated circuit that is dominated by digital circuits that introduce significant amounts of supply noise. For example, a clock-and-data recovery (CDR) phase-locked-loop (PLL) loop may employ a voltage controlled oscillator that is powered by a voltage regulator. Noise in the voltage supply equates to frequency error and ultimately jitter in the generated clock signal.
A conventional low-dropout (LDO) voltage regulator 100 is shown in FIG. 1. The regulator 100 includes a negative feedback amplifier 105 that feeds a pass element 110 at a control node 112, Vmpg. The output of the pass element 110 drives a load 115 connected to an output node 120 at a predetermined voltage level (VOUT). A feedback path is established through a voltage divider 125 including resistors 130, 135. A capacitor 140 is coupled between the output node 120 and ground to provide high frequency noise rejection. The negative feedback amplifier 105 tracks the difference between the output voltage and a reference voltage, VREF, to control the pass element 110 to reduce the voltage error between the inputs of the negative feedback amplifier 105.
The power supply noise rejection ratio (PSRR) of the conventional LDO regulator 100 is shown in FIG. 2. At low noise frequencies, the negative feedback amplifier 105 can force VOUT to track the input reference VREF. The DC supply rejection is limited by the gain of the feedback amplifier 105 and the differential output resistance of the pass element 110. Once the feedback amplifier 105 becomes bandwidth-limited (at a noise frequency of f1), the PSRR begins to degrade as VOUT can no longer track VREF. This degradation is manifested as a zero in the PSRR response. The PSRR continues to degrade with increasing noise frequency until a frequency f2, where the decreasing impedance of the capacitor 140 begins to take effect. At this point, the PSRR improves as high-frequency supply noise at VOUT is attenuated by the capacitor 140.
LDO regulators are limited in supply rejection across a broad noise frequency bandwidth due to limitations in the bandwidth of the feedback amplifier 105, the feedback amplifier gain, the available area for the large capacitor 140 to suppress supply noise beyond feedback amplifier bandwidth, and the output resistance of the pass element.
This section of this document is intended to introduce various aspects of art that may be related to various aspects of the disclosed subject matter described and/or claimed below. This section provides background information to facilitate a better understanding of the various aspects of the disclosed subject matter. It should be understood that the statements in this section of this document are to be read in this light, and not as admissions of prior art. The disclosed subject matter is directed to overcoming, or at least reducing the effects of, one or more of the problems set forth above.